Dipropylene glycol is a compound used for a raw material for polyester resin or polyurethane resin, a raw material for acrylic esters, a hydraulic fluid, an antifreezing liquid, a wetting agent for cellophane, an oil-water compatibilizer, a solvent for printing ink, a raw material for cosmetics, a solvent for flavorants, toiletry solvents, etc. and tripropylene glycol is a compound used for a raw material for polyester resin or polyurethane resin, a raw material for acrylic esters, a solvent for water-soluble oils, a solvent for ink, etc. Dipropylene glycol and tripropylene glycol are known to be produced successfully as a by-product in the production of propylene glycol from propylene oxide and water; for example, Non-Patent Document 1 discloses that dipropylene glycol and tripropylene glycol are formed as by-products in producing propylene glycol by reacting propylene oxide with water and also discloses a step of removing excess water after the reaction.